When hot steam is applied to a steam turbine as working medium, targeted cooling of highly loaded components is desirable in order to increase the steam temperatures which can be reached. Where possible, this targeted cooling encompasses shielding and dissipation of heat through corresponding levels of cooling. In the context of the present application, a steam turbine is to be understood as meaning any turbine or part-turbine through which a working medium in the form of steam flows. By contrast, gas turbines have gas and/or air flowing through them as working medium, but this medium is subject to completely different temperature and pressure conditions than the steam in a steam turbine. Unlike in gas turbines, in steam turbines the working medium flowing to a part-turbine, for example, reaches its highest pressure at the same time as it is at its highest temperature. Therefore, an open cooling system cannot be realized without a cooling medium being supplied from the outside of the part-turbine. It has consequently proven impossible for cooling measures which are known from gas turbines to be transferred to steam turbines in the form which is known for gas turbines and is only suitable for gas turbines.
A casing of a steam turbine is to be understood as meaning in particular the stationary casing component of a steam turbine or part-turbine, which along the axial extent of the steam turbine has an inner space which is intended for the working medium steam to flow through. Depending on the particular type of steam turbine, this may be an inner casing and/or a guide vane carrier. A steam turbine casing is also to be understood as meaning a turbine casing which does not have an inner casing or a guide vane carrier.
A rotor fitted with blades is arranged rotatably along the axial extent in the inner space, so that when heated and pressurized steam flows through the inner space the steam makes the rotor rotate by means of the blades. The blades of the rotor are also known as rotor blades. Furthermore, a steam turbine has stationary guide vanes which penetrate into the spaces between the rotor blades and are held by the inner casing/guide vane carrier. A rotor blade is usually held along an outer side of a steam turbine rotor. It usually forms part of a ring of rotor blades which comprises a number of rotor blades which are arranged along an outer circumference on the outer side of the steam turbine rotor. The main blade part of each rotor blade faces radially outward. A ring of rotor blades is also referred to as a row of rotor blades. A number of rows of rotor blades are usually positioned behind one another. Accordingly, a further, second ring of blades is held along the outer side of the steam turbine rotor at a second location behind the first location along the axial extent.
With the cooling methods which have been disclosed hitherto, in particular for a steam turbine rotor, a distinction has to be drawn between active cooling and passive cooling. In the case of active cooling, cooling is brought about by a cooling medium which is fed to the steam turbine rotor separately, i.e. in addition to the working medium. By contrast, passive cooling is brought about only by suitably guiding or using the working medium in the main flow. Standard cooling of a steam turbine rotor is restricted to passive cooling.
By contrast, it is known from U.S. Pat. No. 6,102,654 and WO 97/49901 for cool steam which has already expanded to flow through a rotor of a steam turbine. In this case, cooling medium is passed through a substantially central cavity along an inner rotor wall and is then fed from there to the outside, in particular to regions of the casing which are to be cooled, via separate radial branch channels. Since the central cavity and the branch channels are arranged at the location where the component is subject to the highest levels of loading, this is highly disadvantageous for the rotor strength. It has the further drawback that a temperature difference across the rotor wall has to remain limited, since otherwise the rotor would be excessively thermally deformed in the event of an excessive temperature difference. For these reasons, a concept of this nature has not yet achieved widespread use. Although heat is dissipated as it flows through the rotor, the dissipation of heat takes place relatively far away from the location where the heat is supplied. Hitherto, it has not been possible to achieve sufficient dissipation of heat in the immediate vicinity of where the heat is supplied.
Further, passive cooling can be achieved by suitably guiding and using the expansion of the steam of the working medium. In this case, the steam which flows to a steam turbine is first of all expanded by exclusively stationary parts, e.g. a ring of guide vanes or radially acting guide vanes, before it is applied to rotating components. In the process, the steam is cooled by approximately 10 K. However, this method can only achieve a very limited cooling action on the rotor.
U.S. Pat. No. 6,102,654 realizes active cooling of a steam turbine rotor to only a very restricted extent, and moreover the cooling is limited to the inflow region of the hot working medium. As shown in FIG. 1 of this application, according to U.S. Pat. No. 6,102,654 cooling medium is passed through the casing onto a protective shield and onto a first ring of guide vanes, in order to reduce the thermal load on the rotor and the first ring of guide vanes. Some of the cooling medium is admixed with the working medium. Aside from the fact that the cooling is restricted to the inflow region, cooling is only supposed to be brought about by flow onto the components which are to be cooled. The cooling effect on the rotor which can be achieved as a result is limited, since it is restricted to the inflow region of the main flow.
It is known from WO 97/49901 for a single ring of guide vanes to be cooled selectively through a separate radial channel in the rotor, fed from a central cavity. For this purpose, cooling medium is admixed with the working medium via the channel, and cooling medium flows selectively onto the ring of guide vanes which is to be cooled. The cooling effect on the rotor is still in need of improvement. Furthermore, the bore disadvantageously increases the rotor stresses significantly compared to the configuration without a bore.
EP 1154123 has described a possible way of removing and guiding a cooling medium from other regions of a steam system and the supply of the cooling medium in the inflow region of the working medium.
To achieve higher efficiency levels in the generation of power using fossil fuels, there is a need to employ higher steam parameters, i.e. higher pressures and temperatures, than has hitherto been customary. In this context, if steam is used as the working medium, pressures of over 250 bar and temperatures of over 540° C. are intended. Steam parameters of this nature are described in detail in the article “Neue Dampfturbinen-konzepte für höhere Eintrittsparameter und längere Endschaufeln” [Novel steam turbine concepts for higher entry parameters and longer end blades] by H. G. Neft and G. Franconville in the Journal VGB Kraftwerks-technik, No. 73 (1993), Volume 5. The content of disclosure of this article is hereby incorporated by reference in the description of the present application. In particular, examples of higher steam parameters are cited in FIG. 13 of the article. In the abovementioned article, a cooling steam supply and passage of the cooling steam through the first guide vane stage and if appropriate also through the second guide vane stage is proposed in order to improve the cooling of a steam turbine rotor. This provides active cooling only for the steam turbine casing. Moreover, the cooling is restricted to the main flow region of the working medium and is still in need of improvement.
Therefore, all the methods which have been disclosed hitherto for cooling a steam turbine rotor, if they are active cooling methods at all, at best provide for a directed flow onto a separate turbine part which is to be cooled and are restricted to the inflow region of the working medium. When higher steam parameters are applied to standard steam turbines, an increased thermal load may result over the entire turbine, and this load could only be alleviated to an insufficient degree by standard cooling of the rotor as described above. Steam turbines which use higher steam parameters in order to achieve higher efficiencies, for example, require improved cooling, in particular of the rotor, in order to sufficiently break down the higher thermal loads on the steam turbine. This gives rise to the problem that when turbine materials which have hitherto been customary are employed, the increasing load on the rotor resulting from increased steam parameters may lead to a disadvantageous thermal load on the rotor and to an unacceptable increase in the temperature of the rotor.